Paper-coating pigment and pigment composition



1940- c. A. JONES ET AL 2,210,835

PAPER-COATING PIGMENT AND PIGMENT COMPOSITION Filed May 25, 1938 POUNDScfismN PER HUNDRED POUNDS Cw C03 R Q I-RaJD TO GIVE. A COATING OFSTANDARD TACK &

TIME OF TREJHTMEINT IN MINUTES amen Wows Chester v9 J0 71/435 PatentedAug. 6, 1940 UNITED STATES PAPER-COATING PIGMENT AND PIGMENT COMPOSITIONChester A. Jones and William J. Montgomery, Hamilton, Ohio, asslgnors toThe Champion Paper and Fibre Company, Hamilton, Ohio, a

corporation of Ohio Application May 25. 1938, Serial No. 210,060

9 Claims.

This invention or discovery relates to improvements in paper-coatingpigments and pigment compositions; and it comprises such a pigment orcomposition, adapted to provide on paper a coat- 5 ing of lowered inkabsorption and improved printing quality, and. comprising finelydivided, precipitated high finishing calcium carbonate which has beensubjected to a mechanical rubbing action until the quantity of adhesiverequired to bind it to paper is substantially reduced; all as morefuliyhereinafter set forth and as claimed.

Calcium carbonate has long been recognized in the paper coating art ashaving a number of highly desirable properties, among which are itsbrilliant white color, high opacity and low cost. Nevertheless itsutilization, while attractive, has not been general. Certain of thecoarser grades have been used in dull finish coated papers, which arehowever only a minor proportion of coated paper production. Theseparticular papers normally have a finish of less than 30 degrees whenmeasured on an Ingersoll glarimeter. Due to the comparatively largeparticle 85 size of this grade of calcium carbonate, it is impossible tocalender this paper to a high degree of smoothness. Consequently halftone printing is not sharp and clear, and furthermore ink absorption isspotty. This grade of calcium carbonate ordinarily requires 10 per centor less of its weight of casein to give a coating which applied tonormal coating stocks and dried and supercalendered in the normal mannerwill just prevent picking (detachment of particles of the a coating bythe ink on the printing plate) when printed with normal half tone inks.

On the other hand certain finely divided, precipitated grades of calciumcarbonate have been proposed for coated paper use which can 40 becalendered to give a smooth high finished product, but experience hasshown that they require excessively large amounts of adhesive to give asufiiciently adherent coating. Also, the coating is often present on theuncalendered ll paper in the form of tiny -mound-like aggregates--acondition associated with a kind of filter cake formation. When suchpaper is supercalendered, it is likely to have a mottled appearance andink absorption is correspondi'ngly non-uniform.

In addition to these two grades of precipitated calcium carbonate thereis known a third grade, of colloidal fineness which, because of itsextreme fineness and excessive absorptive properties, is

55 of no interest in the paper coating art.

The raw'material used in the preparation of ourimproved pigment is acalcium carbonate belonging to the second class above mentioned. That isto say it is a finely divided, high finishing, precipitated calciumcarbonate which (prior to 5 subjection to our process) requires morethan 10 per cent of its weight of casein to give a coating on the usualcoating stocks which after normal supercalendering will print withoutpicking when normal half tone inks are used, and which has a finish inexcess of 30 degrees measured by the Ingersoll glarimeter. Calciumcarbonate conforming to this definition is what is meant throughout thisapplication in referring to finely divided, high finishing, precipitatedcalcium carbonate.

One object of the present invention is to provide a pigment for papercoating compositions which possesses the advantages of high finishingcalcium carbonate, but without requiring the 20 large amounts ofadhesive previously required with this material. Another object is toprovide a finely divided, high finishing, precipitated calcium carbonateof a modified physical form or condition which can be used inconjunction with ordinary adhesives in ordinary amounts and by ordinarymethods ,to give a high grade coated paper having a smooth, relativelyhigh finish surface after supercalendering, substantially free frommottle, having a uniform and somewhat lowered ink absorption andpossessing definitely improved printing qualities for certain kinds ofprinting over paper coated with the untreated material.

In the preparation of our improved pigment 35 the'aforesaid calciumcarbonate is subjected, advantageously in the presence of some water, toa vigorous mechanical rubbing or smearing treatment. It is not actuallyknown that the pigment particles are smeared, but the physical action(it used is one which could produce this result. The general method usedin preparing the pigment is capable of execution in a number ofdifferent modifications, employing different quantities of water,diiferent kinds of apparatus, and different as modes of effecting thedesired mechanical rubbing action upon the calcium carbonate particles.A number of these specific modifications will now be described.

One procedure for carrying out the treatment 5 consists in circulating afluid aqueous suspension of the described calcium carbonate between thefaces of two adjacent discs forced together under moderate pressure, atleast one of which is rotated rapidly. Good results have been obtainedusing an apparatus having for one disc a stationary, smooth, wood blockand for the rotating disc a grooved disc comprising silicon carbide.Other materials for the discs may be used, including rubber, steel, andmarble. The suspension of pigment is recirculated through thisapparatus. Tests may be ,made from time to time as to the amount ofadhesive necessary to add to the pigment to produces coatingcomposition, which when applied to a web of paper in usual ways producesa smooth, adherent coating. The calcium carbonate before treatmentrequires a relatively large proportion of adhesive, e. g., casein, togive good adherence to the web. The proportion required graduallybecomes less as the treatment is continued, and finally, no longerdecreases. At this point we stop the treatment. As a result of thetreatment, the amount of casein required is reduced by 25 per cent ormore as compared with the unrubbed pigment. The treated calciumcarbonate, when madeup with the proportion of adhesive determined bytrial and applied to a web of the usual paper coating stock in the usualway, gives by the usual following operations a smooth-surfaced, uniformcoated paper of brilliant whiteness and outstanding printing qualitiesand having a finish in excess of 30 degrees as measured by the Ingersollglarimeter. It appears that the surfaces of the minute particles ofcalcium carbonate have been modified in some way by the rubbing processwith a change in their relation to the aqueous adhesive. Whatever may bethe explanation, the treatment results in a remarkable improvement ofthe coating.

Instead of smearing fluid calcium carbonate suspension between rotatingdiscs it can be subjected to the action of rapidly rotating metal bladespressing against a stationary disc. Or the suspension may berecirculated through an apparatus similar to a tightly set jordan. Onthe other hand, colloid mills employing a definite clearance betweenrapidly rotating members and depending for their action on shear of aliquid film, are not in general adapted for our purposes. The particlesof the fluid suspension must be brought in contact with two opposingsurfaces and "smeared out under moderate pressure. The desired effect isquite different from a grinding action. In a grinding operation, thepigment particles become progressively smaller so that more adhesive isrequired to bind the pigment to the paper, whereas our rubbing orsmearing action results in a pigment requiring less adhesive.

In addition to the lowered adhesive requirement acquired by the calciumcarbonate as a result of the above described treatment, coated papermade from the product is more uniform in its absorption of printing ink.That is to say the ink lies more evenly on the paper substantially freefrom a spotty or mottled appearance, and this of course enhances itsprinting quality. Also coated paper made from the rubbed product is lessabsorptive to printing ink. That is to say the marked tendency ofprinting ink to rapidly strike into coated paper made from the untreatedcalcium carbonate is considerably reduced by the treatment, especiallyif the rubbing treatment is carried out on the calcium carbonate in thecondition of a paste or dough or as a damp powder. Thus kneading a pastecontaining 60-90 per cent high finishing, precipitated calcium carbonateand 40-10 per cent water in a dough mixer or in a pug mill is veryeffective in reducing the ink absorption of coated paper made from theprodu t.

Edge runners also produce this effect. Passing the calcium carbonate inthe form of a powder between tightly set metal rolls held together bystrong springs is particularly effective for this purpose. In usingrolls we have found that a moisture content of from 5 to 15 per centgives better results than when more or less moisture is present.

The reason for this decreased absorbency towards printing ink is notunderstood. It may be that the calcium carbonate particles becomecapable of being more thoroughly coated with adhesive or perhaps lessadhesive is required to fill in the voids, so that in effect a densersurface or a surface exhibiting a larger proportion of adhesive may beproduced on the coated paper.

In order to obtain paper coating compositions that are fairly high insolids content it is often desirable to add the calcium carbonate insubstantially dry form. -We have found that the method of drying theaqueous slurry of precipitated calcium carbonate greatly affects theproperties of the product as a paper coating pigment.

Thus if the slurry of ordinary precipitated calcium carbonate is allowedto dry without agitation, e. g., tray drying, there is no markeddifference in the adhesive requirements or ink absorption of the calciumcarbonate before and after drying, but if it is subjected to mechanicalagitaiton' during the drying'operation the product requires decidedlyless adhesive to form an adherent paper coating composition and producescoated paper which is definitely less absorbent to printing ink. Forthis purpose we have found that a rotary steam heated kiln dryer is verysatisfactory in providing the desired mechanical rubbing action.

The product resulting from an agitated drying operation or from theother forms of rubbing treatment applied to the calcium'carbonate in theform of a dough, paste, powder, etc., may contain lumps, but these arereadily disintegrated by mixing with water with or without a dispersingagent and ball milling, ink milling, edge running, etc. i

A calcium carbonate suitable for treatment according to the inventionshould be precipitated in finely divided form, and is advantageouslyproduced by precipitation from hydrated lime by carbon dioxide underconditions controlled to give a fine particle size, but other processesmay also be used.

In the accompanying drawing we have shown a chart illustrating theprogressive diminution of the amount of casein required for satisfactoryadhesion, for a particular batch of precipitated calcium carbonatetreated in a particular milling device consisting of a rapidly rotatinggrooved alundum disc pressing against a stationary wood disc. Theprogress of the treatment was gaged by withdrawing samples of the mixfrom time to time and measuring the amount of casein required to give afirmly adherent coating on paper, that is, to give a coating which justfails to pick when printed with an ink of standard tackiness. Beforetreatment 35 pounds casein were required per pounds calcium carbonate.After 100 minutes treatment the amount required was reduced to 25pounds, a reduction of 28.6 per cent. and after 200 minutes it wasreduced to 22.5 pounds, a reduction of 35.7 per cent. Further treatmentin this case resulted in no additional decrease. In this example, thetreatment time for this batch in this apparatius is about 200 minutes.

During the course of treatment the adhesive strength of coatingcompositions made with the pigment increases. There is also observed adecrease in viscosity in the case of fiuid aqueous suspensions. Theshape of the viscosity curve is similar to that of the curve for amountof adhesive required; it has an initial high value, grad- Y uallydecreasing as treatment goes on, to a low, constant value. However, thelow point is reached somewhat sooner than the minimum'of the caseinproportion curve. Hence while viscosity measurements of the suspensionduring treatment give some idea of the progress, the casein consumptiontest described is more significant.

The following example is given as illustrating another modification ofthe treatment. I

A slurry of calcium carbonate was prepared by passing carbon dioxideinto an aqueous suspension of calcium hydroxide and controlling theconditions of precipitation to give a finely divided product. Thisslurry of calcium carbonate was filtered and the filter cake containingabout 42 per cent solids was continuously fed into one end of a rotarydrier at the rate of 2,700 pounds per hour (wet weight). The drier was 6feet in diameter and feet long, rotating at l.5 R. P. M. and wasequipped internally with banks of steam pipes. As the calcium carbonatepassed through the drier it was continually rolled, dropped, andotherwise agitated by the motion of the drier. It was delivered at theother end of the drier as a powder containing about 10 per centmoisture.

This kiln dried calcium carbonate was mixed with about half its weighton the dry basis of the undried slurry of calcium carbonate, caseinsolution, and water to give a composition of 10 parts casein, 100 partscalcium carbonate, and 110 parts water. This mixture was ball milled for2 hours and then recirculated a number of times between the faces of arapidly rotating, grooved, alundum disc pressing against a stationary,wood disc. ment was diluted with waterand applied to normal papercoating stock, 5 pounds of coating (dry basis) being applied per 1000square feet of paper surface. This coated paper was dried andsupercalendered under the standard conditions set forth at the end ofthe specification. This coated paper had a high, uniform finish with agloss of 37.1 degrees as measured on an Ingersoll glarimeter and justwithstood picking when printed with a standard halftone printing ink. Itwas substantially free from mottle.

The original untreated precipitated calcium carbonate, above referredto, when made into a coating mixture with casein and applied to thepaper and dried and supercalendered in the same manner required 23 partsby weight of casein in order to withstand picking on the printing press.The saving in casein is therefore 56 sorption of paper coated with thisproduct in the preparation of coated paper suitable for varnishing andfor gloss ink eilects. Both these grades of paper require a coatingwhich is at the same time smooth and relatively non-absorptive toprinting ink and varnish; the'ink and varnish remain to a considerableextent on the surface and dry to a glossy finish. Coating mixturessuitable for gloss ink and varnishinggrades of coated paper of unusualbrightness may be made by use of the treated calcium carbonate.

The product of this treat-.

Usually some excess casein over the amoimt required to withstand pickingis required for this purpose. A coating mixture suitable for thispurppse was prepared from a finely divided, high finishing,-precipitated calcium carbonate which was first dried in a rotary drier,and then in the form of a 70 per cent aqueous slurry containing a littlegum arable as a dispersing agent, was given a severe, prolonged pebblemill treatment.

This treatment differed from the normal ball. mill treatment often givencoating pigments in' that about twice the ratio of pebbles to pigmentwas used and the treatment was several times as long. To the originalcalcium carbonate it was necessary to add casein to the extent of 25 percent of the dry weight of the calciumcarbonate to just overcome pickingwith a standard halftone ink; after the treatment only 11 per centcasein was required for this purpose.

To produce coating compositions suitable for varnishing and gloss inkgrades of coated paper the pebble milled calcium carbonate was mixedwith 19 per cent casein. When applied to the paper this coating,although containing considerably less casein than would have beenrequired to adequately bind the untreated pigment to the paper wasdecidedly non-absorbent to printing ink and varnish, which'is in markedcontrast to the highly absorbent coated paper made from untreatedcalcium carbonate. Coating compositions suitable for varnishing gradesof coated paper consisting of calcium carbonate and casein in which thecasein is less than 20 per cent of the weight of the calcium carbonateis, so far as we know, new in the art.

The degree of improvement effected by the process depends upon theparticular calcium carbonate used as a raw material and upon theparticular mode of treatment chosen. Those forms of unrubbed calciumcarbonate which require less than 10 per cent of casein for adequatebonding to the paper are in general too coarse to be useful for thispurpose while those of colloidal fineness, i. e., those which requiremore than per cent of casein do not give suitable coatings. Usingcalcium carbonate intermediate these two grades, a reduction in caseinof at least 25 per cent of that required for the untreated calciumcarbonate may be expected by the rubbing treatment.

While the invention has been described with reference to casein as theadhesive, the same improvements result when other aqueous adhesives areused, such as starchf the binder requirement lessens.

As is well known in the art the finish of a coated paper is affected bythe-severity of the supercalendering operation, per cent moisture in thecoated paper, etc. In order to define our product as exactly as possiblewe have selected certain standard calendering conditions for. testingour product. These conditions are as follows:

' A normal weight of coating (e. g., 5 pounds dry weight per side per1000 square feet of paper) is applied to the usual paper coating stocks,dried in equ librium with air at 50 per cent relative humidity. andcalendered at a pressure of 600 pounds per linear inch, by passingthrough the top nip of a three-roll supercalender consisting of top andbottom rolls of chilled iron, 12 inches in diameter, and middle roll ofcompressed cotton, 16.5 inches in diameter, on a 7.25-inch core, pressedto a hardness of 85 measured on a Shore densometer, Type D, with thecoated side against the metal, twice in a direction parallel to thegrain of the paper and twice at right angles to the grain, at a speed of45 feet per minute. The calcium carbonate of our invention when mixedwith a quantity of casein sumcient to give a coated paper which willJust all to pick when printed with a standard half tone ink, will befound to require between 7.5 and of its weight of casein and thiscoating composition when tested under the conditions specified abovewill give a coated paper having a gloss in excess of degrees as measuredby the Ingersoll glarimeter.

Our rubbed calcium carbonate may be used alone with adhesive or inadmixture with other pigments, but for the purposes of this testa-special test composition containing only the carbonate with casein isused.

This application is a continuation-in-part of Serial No. 2430, filedJanuary 18, 1935.

What we claim is:

i. Pigment for coated paper comprising finely divided precipitatedcalcium carbonate which is characterized in that when mixed with asuffiby the Ingersoll glarimeter and will just withstand picking orlifting when printed with the usual half tone inks, said quantity ofcasein being between 7.5 per cent and 25 per cent of the weight of thecalcium carbonate and said calcium carbonate further characterized inthat it is prepared by subjecting finely divided calcium carbonate to'amechanical rubbing action until the amount of casein required to produceany predetermined strength of bond between the calcium carbonate andpaper is reduced at least 25 per cent below the amount required prior tothe rubbing treatment.

' 2. The pigment of claim 1 further characterized in that the calciumcarbonate is prepared by reacting hydrated lime with carbon dioxideunder conditions adapted to give a finely divided precipitate.

3. Paper-coating composition which comprises prises finely dividedprecipitated calcium carbonate 100 parts by weight and casein 7.5 to 25parts by weight characterized in that when applied to the usual papercoating stocks and dried and supercalendered in the normal manner givesa coated paper which has a gloss in excess of 30 degrees as measured bythe Ingersoll glarimeter and which prints without lifting or pickingwhen printed with the usual half tone inks, and further characterized inthat the calcium carbonate is prepared by subjecting it to a mechanicalrubbing action until the amount of casein required to produce anypredetermined strength of bond between the unprocessed calcium carbonateand paper is reduced at least 25 per cent.

4. Pigment for coated paper which comprises finely divided precipitatedcalcium carbonate characterized in that when mixed with a sumcientquantity of casein and water, and five pounds dry weight of theresulting composition, per 1000 square feet, are applied to one surfaceof the usual paper coating stocks, it gives a coated paper which whendried in equilibrium with air at 50 per cent relative humidity andcalendered at a pressure of 600 pounds per lineal inch, by passingthrough the top nip of a threeroll supercalender consisting of top andbottom rolls of chilled iron 12 inches in diameter and a middlecompressed cotton covered roll 16% inches in diameter on a 7%-inch corepressed to a hardness of 85 measured on a Shore densometer, type D, withthe coated side against the metal roll, twice in a direction parallel tothe grain of the paper and twice at right angles to the grain at a speedof feet per minute, has a gloss in excess of 30 degrees as measured bythe Ingersoll glarimeter and Just withstands picking when printed withthe usual half tone inks, said quantity of casein being between 7.5 percent and 25 per cent of the weight of the calcium carbonate and saidcalcium carbonate further characterized in that it is prepared bysubjecting it to a mechanical rubbing action until the amountof caseinrequired to form a coating on paper which will Just withstand pickingwhen printed with a normal half tone ink is reduced at least 25 per centbelow the amount required prior to the rubbing treatment.

5. The pigment of claim 4 in which the mechanical rubbing'actioncomprises rubbing the finely divided, precipitated calcium .carbonate inthe presence of more than 5 per cent water.

6. The pigment of claim 4 in which the mechanical rubbing actioncomprises drying the calcium carbonate under conditions of agitation.

7. The pigment of claim 4 further characterized in that the calciumcarbonate is prepared by reacting hydrated lime with carbon dioxideunder conditions adapted to give a finely divided precipitate.

8. Pigment for coated paper which comprises finely divided precipitatedcalcium carbonate characterized in that it has been subjected to amechanical rubbing treatment in the presence of more than 5 per cent ofwater and in that when mixed with a suillcientquantity of casein andwater, and five pounds dry weight of the resulting composition, per 1000square feet, are applied to one surface of the usual paper coatingstocks, itgives a coated paper which when dried in equilibrium with airat per cent relative humidity and calendered at a pressure of 600 poundsper lineal inch, by passing through the top nip of a three-rollsupercalender consisting of top and bottom rolls of chilled iron 12inches in diameter and a middle compressed cotton covered roll 165inches in diameter on a 7 -inch core pressed to a hardness of measuredon a Shore densometer, type D, with the coated side against the metalroll, twice in a direction parallel to the grain of the paper and twiceat right angles to the grain at a speed of 45 feet per minute, has agloss in excess of 30 degrees as measured by the Ingersoll glarimeterand just withstands picking when printed with the usual half tone inks,said quantity of casein being between 7.5 per cent and 25 per cent ofthe weight of the calcium carbonate, and being at least 25 per cent lessthan is required with the same pigment in its untreated conditlon.

9. The pigment of claim 4 in which the mechanical rubbing comprisesdrying the finely divided, precipitated calcium carbonate underconditions of agitation and subsequently rubbing it in the presence ofmore than 5 per cent of water.

CHESTER A. JONES. WILLIAM J. MONTGOMERY.

